Staphylococcus aureus is a Gram-positive spherical bacterium. Annual US mortality exceeds that of any other infectious disease, including HIV/AIDS, and S. aureus is the leading cause of bloodstream, lower respiratory tract, skin & soft tissue infections. There is currently no authorised vaccine. A vaccine based on a mixture of surface polysaccharides from bacterial types 5 and 8, StaphVAX™, failed to reduce infections when compared to the placebo group in a phase III clinical trial in 2005.
Reference 1 reports that the “V710” vaccine from Merck and Intercell is undergoing a phase 2/3 trial on patients undergoing cardiothoracic surgery. The V710 vaccine is based on a single antigen, IsdB [2], a conserved iron-sequestering cell-surface protein.
S. aureus causes a range of illnesses from minor skin infections to life-threatening diseases such as pneumonia, meningitis, osteomyelitis, bacteremia, endocarditis, toxic shock syndrome, organ abscesses and septicemia. The bacterium has multiple virulence factors which are differentially expressed during different phases of its life cycle, and so a vaccine which can prevent one disease might not prevent another. For instance, the V710 vaccine may be effective against hematic spread of the S. aureus, but may be ineffective against pneumonia. One aim of the invention is to provide vaccines which can protect against hematic spread and pneumonia, and which may also elicit an opsonic response.
Thus there remains a need to identify further and improved antigens for use in S. aureus vaccines, and in particular for vaccines which are useful against multiple S. aureus pathologies.